1. Field of the Invention
This invention relates to a plasma display panel (PDP), and more particularly to a dielectric for an upper plate suitable for the PDP and a method of fabricating the same. The present invention also is directed to a dielectric for a lower plate in the PDP and a dielectric composition adaptive for forming a barrier rib in the PDP.
2. Description of the Related Art
Generally, a plasma display panel (PDP) radiates a fluorescent body by an ultraviolet with a wavelength of 147 nm generated during a discharge of He+Xe or Ne+Xe gas to thereby display a picture including characters and graphics. Such a PDP is easy to be made into a thin film and large-dimension type. Moreover, the PDP provides a very improved picture quality owing to a recent technical development. The PDP is largely classified into a direct current (DC) driving system and an alternating current (AC) driving system.
The PDP of AC driving system is expected to be highlighted 30 into a future display device because it has advantages in the low voltage drive and a prolonged life in comparison to the PDP of DC driving system. Also, the PDP of AC driving system allows an alternating voltage signal to be therebetween to generate a discharge every half-period of the signal, thereby displaying a picture. Since such an AC-type PDP uses a dielectric material, the surface of the dielectric material is charged with electricity. The AC-type PDP allows a memory effect to be produced by a wall charge accumulated to the dielectric material due to the discharge.
FIG. 1 is a sectional view showing the structure of a discharge cell in the conventional three-electrode AC-type PDP, in which a lower plate is illustrated in a state of rotating an angle of 90xc2x0. In FIG. 1, the discharge cell includes an upper plate 10 provided with a sustaining electrode pair 12 and 14, and a lower substrate 20 provided with an address electrode 20. The upper substrate 10 and the lower substrate 20 are spaced, in parallel, from each other with having a barrier rib 28 therebetween.
A mixture gas such as Nexe2x80x94Xe or Hexe2x80x94Xe, etc. is injected into a discharge space defined by the upper substrate 10 and the lower substrate 20 and the barrier rib 28. The sustaining electrode pair 12 and 14 consists of transparent electrodes 12A and 14A and metal electrodes 12B and 14B. The transparent electrodes 12A and 14A are usually made from Indium-Tin-Oxide (ITO) and has an electrode width of about 300 xcexcm. Usually, the metal electrodes 12B and 14B take a three-layer structure of Crxe2x80x94Cuxe2x80x94Cr and have an electrode width of about 50 to 100 xcexcm. These metal electrodes 12A and 14A play a role to decrease a resistance of the transparent electrodes 12A and 14A6 with a high resistance value to thereby reduce a voltage drop. Any one 12 of the sustaining electrode pair 12 and 14 is used as a scanning/sustaining electrode that responds to a scanning pulse applied in an address interval to cause an opposite discharge along with the address electrode 22 while responding to a sustaining pulse applied in a sustaining interval to cause a surface discharge with the adjacent sustaining electrodes 14. A sustaining electrode 14 adjacent to the sustaining electrode 12 used as the scanning/sustaining electrode is used as a common sustaining electrode to which a sustaining pulse is applied commonly. A distance between the sustaining electrode pair 12 and 14 is set to be approximately 100 xcexcm. On the upper substrate 10 provided with the sustaining electrode pair 12 and 14, an upper dielectric layer 16 and a protective layer 18 are disposed. The dielectric layer 16 is responsible for limiting a plasma discharge current as well as accumulating a wall charge during the discharge. The protective film 18 prevents a damage of the dielectric layer 16 caused by a sputtering generated during the plasma discharge and improves an emission efficiency of secondary electrons. This protective film 18 is usually made from MgO. The address electrode 22 is crossed with the sustaining electrode pair 12 and 14 and is supplied with a data signal for selecting cells to be displayed. On the lower substrate 20 formed with the address electrode 24, a lower dielectric layer 24 is provided. Barrier ribs 28 for dividing the discharge space are extended perpendicularly on the lower dielectric layer 24. On the surfaces of the lower dielectric layer 24 and the barrier ribs 28 is coated a fluorescent material 26 excited by a vacuum ultraviolet lay to generate a red, green, or blue visible light.
In such a PDP, the upper dielectric layer 16 has a transmissivity of about 85% at the central wavelength to transmit a visible light. The upper dielectric layer 16 also accumulates a wall charge to thereby sustain the discharge by a discharge sustaining voltage. In this case, since a larger capacitance value is required to lower a discharge voltage, the upper dielectric layer 16 has a relatively high dielectric constant of about 10 to 15. The upper dielectric layer 16 plays a role to protect the sustaining electrodes 12 and 14 from an ion impact during the plasma discharge and serves as an anti-diffusion film. The upper dielectric layer 16 consists of first and second upper dielectric layers 16A and 16B that are usually made from a glass having a different softening point. As the first upper dielectric layer 16A contacted directly with the sustaining electrodes 12 and 14 is used a glass with a relatively higher softening point so as to avoid a chemical reaction between the transparent electrodes 12A and 14A and the metal electrodes 12B and 14B. The second upper dielectric layer 16B formed on the first upper dielectric layer 16A requires a high smoothing coefficient so as to provide the protective film 18. For this reason, as the second upper dielectric layer 16B is used a low softening glass having a softening point tens of degrees lower than the first upper dielectric layer 16A.
FIG. 2 shows a process of forming the upper dielectric layer 16. At step S2, a first glass paste with a relatively high softening point is printed on the upper substrate 10 provided with the sustaining electrodes 12 and 14 using the screen printing technique. In this case, the glass paste is prepared by mixing borosilicate glass powder having a particle diameter of 1 to 2 xcexcm and containing Pb of about more than 40% with an organic binder. At step S4, the printed first glass paste is fired at a temperature of 550 to 580xc2x0 C. to form the first upper dielectric layer 16A. Then, at steps S6 and S8, a second glass paste with a relatively low softening point is printed on the first upper dielectric layer 14A using the screen printing technique and thereafter is fired at a temperature of 550 to 580xc2x0 C., thereby forming the second upper dielectric layer 16B.
As described above, the upper dielectric layer 16 is provided by firing a paste, which is a mixture mixed with an organic binder, at a temperature of less than 600xc2x0 C. so as to prevent a thermal deformation of the upper substrate 10. Due to this, since the conventional upper dielectric layer 16 fails to become a complete plastic material, a bubble caused by a residual organic material exists in the interior thereof. The bubble existing in the interior of the dielectric layer brings about an insulation destruction to have a serious influence on a characteristic and a life of the device. A bubble generating at a contact portion between the upper dielectric layer 16 and the sustaining electrodes 12 and 14 causes a problem in that it drops a dielectric constant to increase a discharge voltage. Furthermore, the conventional upper dielectric layer 16 has a problem in that a glass component resulting from a diffusion caused by a thermochemical reaction at a portion contacting the sustaining electrodes 12 and 14 upon firing is penetrated into the sustaining electrodes 12 and 14 to raise a resistance value of the sustaining electrodes 12 and 14 and thus increase a discharge voltage.
The lower dielectric layer 24 prevents atom diffusion from the address electrode 22 into the fluorescent material 26. The lower dielectric layer 24 must reflect a visible light back-scattered and coming out from the fluorescent material 26 to prevent a brightness deterioration of the PDP caused by a back light. The barrier rib 28 also must reflect a visible light back-scattered and coming out from the fluorescent material 26 like the lower dielectric layer 24 to prevent an optical interference between discharge cells as well as to prevent a brightness deterioration caused by a back light. Accordingly, the lower dielectric layer 24 and the barrier rib 28 require a dense organization to have a high reflectivity. To this end, as the lower dielectric layer 24 and the barrier rib 28 is used a glass-ceramics material mixing the same series of parent glass with an oxide filler for increasing the reflectivity.
In other words, most materials for the barrier rib 28 and the lower dielectric layer 24 uses a glass-ceramics material in which borosilicate glass powder containing Pb of about more than 40% is mixed with an oxide filler consisting of 10 to 30 weight % TiO2 powder or 10 to 30 weight % Al2O3 powder with a particle size of 1 to 1 xcexcm. In this case, the relationship of a composition of an oxide filler for the lower dielectric layer 24 and the barrier rib 28 to characteristics of the lower dielectric layer 24 and the barrier rib 28 and thus to a characteristic of the PDP is indicated in the following tables:
It can be seen from Table 1 and Table 2 that, when 10 to 30 weight % TiO2 or 10 to 30 weight % Al2O3 is used as an oxide filler, the barrier rib 28 and the lower dielectric layer 24. Accordingly, as indicated in Table 3, this causes a problem in that, since a large amount of back light transmits the barrier rib 28 and the lower dielectric layer 24, the brightness of PDP device becomes low. In order to solve this problem, the lower dielectric layer 24 and the barrier rib 28 require a reflection characteristic of more than 50% at the central wavelength, a low dielectric constant of less than 10 and a dense organization. Also, the lower dielectric layer 24 and the barrier rib 28 require a low thermal expansive coefficient for preventing a crack, a thermal stability and a low firing temperature for preventing a crack in the lower substrate 20 upon firing.
Accordingly, it is an object of the present invention to provide an upper dielectric layer in a PDP and a fabrication method thereof that is capable of preventing an insulation breakdown and a crack caused by a bubble generation resulting from an incomplete firing and a residual organic material from the upper dielectric layer by using a ferrodielectric thin film.
A further object of the present invention is to provide an upper dielectric layer in a PDP that is capable of reducing a discharge voltage by using a ferrodielectric thin film.
A yet further object of the present invention is to provide an upper dielectric layer in a PDP and a fabrication method thereof that is capable of preventing a thermochemical reaction to electrodes upon firing of the upper dielectric layer.
A still further object of the present invention is to provide dielectric compositions for a lower dielectric layer and a barrier rib in a PDP that is capable of increasing the reflectivity of the upper dielectric layer and the barrier to improve the brightness.
A still further object of the present invention is to provide dielectric compositions for a lower dielectric layer and a barrier rib in a PDP that is capable of decreasing the dielectric constant of the lower dielectric layer and the barrier rib to improve a response speed of the PDP.
A still further object of the present invention is to provide dielectric compositions for a lower dielectric layer and a barrier rib in a PDP that is capable of increasing a degree of crystallization of a dielectric material to prevent an atom diffusion from an address electrode into a fluorescent material.
In order to achieve these and other objects of the invention, an upper dielectric layer in a plasma display panel according to one aspect of the present invention includes a ferrodielectric thin film formed on an upper substrate provided with a certain electrodes; and a dielectric thick film formed on the ferrodielectric thin film. Also, the upper dielectric layer further includes a ferrodielectric thin film formed on the dielectric thick film.
A process of fabricating an upper dielectric layer in a plasma display panel according to another aspect of the present invention includes the steps of forming a ferrodielectric thin film on an upper substrate provided with a certain electrodes using a vacuum vapor deposition technique; and forming a dielectric thick film on the ferrodielectric thin film using a screen printing technique. Also, the process further includes forming a ferrodielectric thin film on the dielectric thick film using the vacuum vapor deposition technique.
A dielectric composition for a lower dielectric layer and a barrier rib in a plasma display panel according to still another aspect of the present invention includes a parent glass; and an oxide filler containing a phosphorus (P) element. Also, the dielectric composition further includes another oxide filler made from TiO2.